The present invention relates to nuclear magnetic resonance (MR) imaging and more particularly to a novel method of three-dimensional MR magnitude contrast or "time-of-flight" angiographic imaging acquisition of several overlapping thin slabs.
Non-invasive MR imaging techniques can be utilized to detect flowing fluids, such as blood in the human body. Techniques such as MR angiography provide a selective means of fluid flow detection in blood vessels. The bifurcations of the carotid arteries present one area of the human body in which blood flow information is crucial for diagnostic use. The methods described in this specification can be applied equally well to other areas and to subjects other than the human body. The bifurcations of the carotid arteries are frequent sites of atheromatous plaque formation, which is itself a precursor of transient ischemic attack and stroke. It is therefore highly desirable to be able to image atherosclerotic plaque before ulcerization which may result in a brain embolism. The presence of smooth atherosclerotic plaque can cause hemodynamically significant stenosis. The presence of the plaque may be evaluated with a standard, invasive, X-ray dye angiographic technique which requires injection of a contrast dye, via a catheter. This method is not totally benign. It is therefore highly desirable to use a non-invasive, magnetic resonance technique which provides a display in which only the arterial blood flow appears, while the stationary surrounding tissue is suppressed. It is also desirable to determine blood flow velocity and direction.
A nuclear magnetic signal from flowing blood can be obtained in one of two ways using existing MR techniques. The signal can be caused by a velocity dependent phase shift due to motion of the blood. This method is called phase contrast imaging. In the alternative, the signal can be caused by the relative increase in a nuclear magnetic signal magnitude due to the inflow of fresh, non-saturated, magnetization into the imaging region. Thus, this method is called magnitude contrast or "time-of-flight" imaging. Existing MR techniques have limited spacial resolution which is caused by an inherently low signal-to-noise ratio. Existing techniques also have flow effects which frequently cause artifactual loss of signal intensity. Thus, present MR time-of-flight, or magnitude contrast, imaging techniques can provide important flow information, but have disadvantages associated with the method of acquisition.
MR techniques can further be classified as either two-dimensional projection (similar to conventional X-ray angiography) or three-dimensional. Two dimensional projection imaging techniques, which directly acquire a projection through the subject are relatively fast, but generate only a single view of the areas of interest. Further acquisitions are necessary to obtain additional views. More significantly, all projection techniques are very sensitive to phase dispersion along the projection direction and there is generally significant signal loss due to this phase dispersion.
It is possible to combine many two dimensional images to make a multiple thin slice image. Multiple thin slice (two-dimensional) magnitude contrast techniques of this type obtain reasonable images of large and small vessels. However, the images are very noisy because of the small number of signal measurements which are used to generate the image relative to the number of signal measurements used in 3D imaging. There is also signal loss from velocity dependent phase dispersion due to the large slice thickness. In two dimensional magnitude contrast imaging, the slice thickness is typically not less than 2 mm. There is a trade off between slice thickness and echo time ,T.sub.E (time from the center of the central lobe of a radio frequency (RF) pulse to the center of the received echo) and signal is lost due to either long echo time or large slice thickness. Each three dimensional volume, for which a single measure of the MR signal is obtained for display as part of the image, is called a voxel. Averaging the MR signal over an elongated voxel dimension, such as a 2-3 mm slice thickness, in which the fluid motion is not uniform, causes a velocity dependent loss in signal. This signal loss is due to phase dispersion. Signal loss therefore will occur in voxels where the dimensions of the voxel contain the edges of a vessel or a curved vessel segment where large spatially dependent variations in velocity occur. Therefore, it is advantageous to use a small voxel volume in order to obtain a good, usable signal from the inflowing blood.
Direct three-dimensional acquisition magnitude contrast techniques have the disadvantage of low blood signal due to the thickness of the three dimensional slab imaged. Due to the thickness of the slab imaged, blood remains in the slab for a significant fraction of the imaging time. Blood that remains in the slab is saturated by a RF pulse used in MR imaging. The saturation of the blood causes its signal to be weaker than the signal from fresh inflowing blood. This decrease in signal is especially true for small vessels with slow blood flow. The loss in vessel detail due to the time blood spends in the slab, as well as due to phase dispersion across the voxel dimensions, is reduced with the use of thinner slabs and thus fewer and/or smaller voxels in the acquisition. However, this decreased slab thickness results in a thinner field of view which limits the diagnostic utility of the technique.